No, there are no additional fees when using Touch 'n Go eWallet for your Huawei AppGallery purchases
Below are the articles that may be useful for you :
How do I make payments using Huawei AppGallery auto debit with Touch 'n Go eWallet?
How do I check my Huawei AppGallery purchase history?
Can Touch 'n Go eWallet be used for Huawei AppGallery in other countries?
What is the wallet limit for Huawei AppGallery auto debit with Touch 'n Go eWallet?